1. Field of the Invention
The present invention relates to an operational amplifier (op amp) circuit, and more particularly, to an operational amplifier circuit with a controllable intermediate circuitry set therein.
2. Description of the Prior Art
The operational amplifier (op amp) is an essential circuit building block of universal importance. A reason for the popularity of the op amp is its versatility; a great amount of operations can be done with op amps. Normally, an operational amplifier circuit is used to drive a load according to an analog input signal. However, if a digital input signal is adopted, a digital-to-analog converter (DAC) must be set in the signal input end of the operational amplifier circuit to convert the digital input signal into an analog input signal. The operational amplifier circuit can then drive the load according to the analog input signal. For instance, the liquid crystal display (LCD) source driver is a kind of device that needs to utilize an operational amplifier circuit including a DAC to drive a LCD display unit according to a digital input signal.
FIG. 1 shows a schematic diagram of a conventional operational amplifier circuit 100 utilized in a LCD source driver. The operational amplifier circuit 100 shown in FIG. 1 includes a DAC 110 and an op amp 120. The LCD display unit driven by the operational amplifier circuit 100 is represented by a block “load 190” in FIG. 1. The DAC 110 converts a digital input signal SIN of the LCD source driver into an analog input signal S′IN. The op amp 120 then drives the load 190 according to the analog input signal S′IN.
For the conventional operational amplifier circuit 100 shown in FIG. 1 to drive the load 190 moderately, the actual loading induced by the load 190 must be matched with the driving ability of the operational amplifier circuit 100. Herein the term “driving ability” can refer to charging current and discharging current providing ability of the operational amplifier circuit 100. The term “driving ability” can also refer to load charging and discharging speed of the operational amplifier circuit 100. When the actual loading induced by the load 190 is too small for the operational amplifier circuit 100 to drive, the load 190 might be over driven. An output voltage VOUT of the operational amplifier circuit 100 tends to become unstable and oscillating. When the actual loading induced by the load 190 is too large for the operational amplifier circuit 100 to drive, the load 190 might be under driven. It might become difficult for the operational amplifier circuit 100 to correctly adjust the level of the output voltage VOUT within a required time constraint.